Abstract: A new electrochemical method has been developed for selective etching of semi-insulating QaAs (gallium arsenide) using citric acid-hydrogen peroxide-water system. The effects of electrolyte composition, etching time and current density on the etching rate are studied. The etched surfaces are also examined by SEM. The experimental results show that a smooth surface and flatbottomed holes can be obtained with this method. Based on simple faradic analysis, it is found that the effect of DO current is to make the surface smooth, but the dissolution mechanism of GaAs in electrochemical etching is, probably, mainly due to chemical reaction.